Concerning the structure of hydrogen molybdenum bronze phase III.: A combined theoretical -: experimental study

The structure of hydrogen molybdenum bronze phase III, HMoO3 (X approximate to 5/3) has been studied on the basis of a combined experimental-theoretical approach. Our study of the average structure allows us to qualify previous contradictory results concerning the octahedral distortions and show no sign of hydrogen occupation of the intralayer sites. A first-principles theoretical study of this average structure leads to a clear understanding of the nature of the band structure as well as the importance and requirements for metal-metal bonding. The nature of the calculated Fermi surface suggests that a 3b x 2c superstructure may be a better description for phase 111, which would be compatible with powder X-ray diffraction (XRD) results. Since the information available from powder XRD data does not allow a direct determination of the superstructure, the problem has been approached by means of first-principles geometry optimizations and molecular dynamics studies. The main structural features of this superstructure (distribution of Mo-Mo bonds, main distortions of the Mo-O network and hydrogen distribution) are described.